Vertical-cavity lasers (VCLs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications. Further, VCLs emit a generally circular beam that is well suited for coupling to optical fibers. Single mode VCLs have additional advantages in terms of high-speed data transmission due to the well-behaved properties of the single optical mode.
Many approaches for controlling the transverse mode structure have been developed in recent years. Methods of mode control fall into two general categories, gain/loss modulation and index modulation. In the first category, the imaginary part of the refractive index is tailored laterally so as to provide more gain or less loss for the fundamental mode with respect to higher order modes. An example of gain modulation is the use of a current constriction element, such as an oxide or implant aperture, to preferentially pump the fundamental mode.
Similarly, there are a variety of methods that may be used to provide loss modulation. For example, a contoured mirror may be used to increase reflectivity for the fundamental mode or decrease reflectivity for higher order modes. Alternatively anti-phasing of a mirror may be used to increase transmission losses for higher order modes to provide selective loss modulation. Further, the optical cavity may be extended to increase diffraction losses for higher order modes, or selective mirror doping may be used to increase absorption losses for higher order modes.
Index modulation techniques, by contrast, generally tailor the real part of the refractive index laterally so as to form a waveguide. Methods of index modulation include lateral regrowth of lower index material, oxide apertures, and effective index guiding via resonant cavity wavelength modulation. The waveguide in the index modulation methods typically requires a region of higher index surrounded by a region of lower index. The relative index step determines the radius of the waveguide for single mode operation via                               r          =                                    2.405              ⁢              λ                                      2              ⁢              π              ⁢                                                                    n                    1                    2                                    -                                      n                    2                    2                                                                                      ,                            (                  Eq          .                                           ⁢          1                )            where λ is the lasing wavelength and n1 and n2 are the effective indexes of refraction in the core and cladding regions, respectively. As seen from the equation the greater the index step between the effective indices of refraction, the smaller the single mode cutoff radius.
The conventional oxide aperture transverse mode control method entails a relative high index step and therefore requires a relatively small electrical and optical aperture to achieve single mode operation. In operation the thermal impedance, voltage, resistance and operating current density all increase with decreasing aperture diameter. In addition, the diffraction loss from an intra-cavity or surface aperture (whichever is present) and the divergence angle also increase with decreasing aperture resulting in a corresponding decrease in output power.
To date only extended cavity designs have been proposed to increase the aperture diameter of single mode devices. However, extended cavity designs may allow additional longitudinal modes to lase thereby limiting the single mode range of operation. In addition, extending the optical cavity tends to reduce the photon density in the longitudinal direction, thus reducing the achievable modulation bandwidth.
In addition to small device size, one other problem plagues vertical cavity lasers. VCLs may be subject to thermal lensing created by the local heating of the gain region due to electrical power dissipation. The attendant rise in junction temperature raises the index of the material in the core region and provides additional optical index guiding. The additional index step, on the order of 0.015 to 0.025, may be sufficient to cause multimode operation. As a result, the electrical and/or optical apertures must be constricted further, or the loss modulation increased, to maintain single mode operation.